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MIR Free-Space Isolators (2.20 - 9.80 µm)
2.5 µm Center Wavelength
4.5 µm Center Wavelength
9.4 µm Center Wavelength
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Our Adjustable Narrowband Isolators can be tuned to maximize the peak isolation for any wavelength within a narrow spectral range (shaded in this graph based on theoretical data). Please see the Wavelength Tuning tab for more details.
Thorlabs' Free-Space Adjustable Narrowband MIR Optical Isolators are designed for use in the MIR spectral range (2.20 - 9.80 µm). Optical isolators, also known as Faraday isolators, are magneto-optic devices that preferentially transmit light along a single direction, shielding upstream optics from back reflections. Back reflections can create a number of instabilities in light sources, including intensity noise, frequency shifts, mode hopping, and loss of mode lock. In addition, intense back-reflected light can permanently damage optics. Please see the Isolator Tutorial tab for an explanation of the operating principles of a Faraday isolator.
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These isolators are factory aligned to provide peak isolation at the specified center wavelength. They also offer the user the ability to adjust the alignment of the output polarizer with respect to the input polarizer, allowing the peak of the isolation curve to lie anywhere within a 50 - 200 nm range; see the tables below for details. Please see the Isolator Types tab for additional design details and representative graphs of the wavelength-dependent isolation.
Each isolator's housing is marked with an arrow that indicates the direction of forward propagation. In addition, all isolators have engravings that indicate the alignment of the input and output polarizers. The isolators can be mounted in a variety of adapters; compatible mounting options are listed with each product as well as at the bottom of the page.
Thorlabs also manufactures isolators for fiber optic systems and wavelengths extending down to the UV (see the Selection Guide table to the left). If Thorlabs does not stock an isolator suited for your application, please refer to the Custom Isolators tab for information on our built-to-order options, or contact Tech Support. Thorlabs' in-house manufacturing service has over 25 years of experience and can deliver a free-space isolator tuned to your center wavelength from 244 nm to 9.80 µm. Our vertically integrated manufacturing structure allows us to offer Faraday rotators used in optical isolators. We offer a selection of these products from stock and can provide custom Faraday rotators upon request.
The shaded region on each graph below represents the wavelength tuning range of the isolator. The peaks of the isolation and transmission curves can be shifted anywhere within this range by rotating the output polarizer, as detailed in the Wavelength Tuning tab. These curves show theoretical data; isolation and transmission will vary from unit to unit.
Tuning an Adjustable Narrowband Isolator
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When the isolator is tuned away from its design wavelength, the maximum transmission falls because the output polarizer's transmission axis is not parallel to the polarization direction of the output light.
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Our Adjustable Narrowband Isolators can be tuned to maximize the peak isolation for any wavelength within a narrow spectral range (shaded in this graph).
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Light Not at the Design Wavelength is Partially Transmitted
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Light at the Design Wavelength is Rejected
Operating Principles of Optical Isolators
The magnitude of the rotation caused by the Faraday rotator is wavelength dependent. This means that light with a different wavelength than the design wavelength will not be rotated at exactly 45°. For example, if 4.5 µm light is rotated by 45° (that is, 4.5 µm is the design wavelength), then 4.45 µm light is rotated by 45.4°. If 4.45 µm light is sent backward through an isolator designed for 4.5 µm without any tweaking, it will have a net polarization of 45° + 45.4° = 90.4° relative to the axis of the input polarizer. The polarization component of the light parallel to the input polarizer's axis will be transmitted, and the isolation will therefore be reduced.
Since the net polarization needs to be 90° to obtain the maximum isolation, the output polarizer is rotated to compensate for the extra rotation being caused by the Faraday isolator. In our example, the new polarizer angle is 90° - 45.4° = 44.6°. This adjustment increases the isolation back to the same value as at the design wavelength.
Consequences of Wavelength Tuning Procedure
Here, θ is the angle between the polarization direction of the light after the Faraday rotator and the transmission axis of the polarizer, I0 is the incident intensity, and I is the transmitted intensity. The decrease in transmission is very slight for our mid-IR isolators. In our example (a 50 nm difference between the design wavelength and the usage wavelength), θ = 45.4° - 44.6° = 0.8°, so I = 0.9998 I0. This case is shown in the graphs above.
Thorlabs' isolator housings make it easy to rotate the output polarizer without disturbing the rest of the isolator. Our custom isolator manufacturing service (see the Custom Isolators tab) can also provide an isolator specifically designed for a particular center wavelength. These custom isolators are provided at the same cost as their equivalent stock counterparts. For more information, please contact Technical Support.
Illustrated Tuning Procedure
To optimize the isolation curve for a specific wavelength within the tuning range, the alignment of the output polarizer may be tweaked following the simple procedure outlined below. Only a minor adjustment is necessary to cover a range of tens of nanometers. An isolator need only be fine-tuned once it has undergone its mandatory initial alignment. The fine-tuning process differs slightly for different isolator packages, but the principle remains the same across our entire isolator family, and complete model-specific tuning instructions ship with each isolator.
Secure the isolator in the mounting adapter by tightening the locking screw hex on the adapter. Note that the isolator should be secured in a way that exposes the polarizer tuning setscrew.
The isolator is mechanically stable in this position as long as the isolator has not been brought forward too much. It should therefore not be necessary to reinsert the isolator at the end of the tuning procedure.
As long as the isolator was not brought forward too much at the end of Step 3, the isolator will be mechanically stable in this position. Attempting to reinsert the isolator at this point may cause misalignment.
Fixed Narrowband Isolator
The isolator is set for 45° of rotation at the design wavelength. The polarizers are non-adjustable and are set to provide maximum isolation at the design wavelength. As the wavelength changes the isolation will drop; the graph shows a representative profile.
Adjustable Narrowband Isolator
The isolator is set for 45° of rotation at the design wavelength. If the usage wavelength changes, the Faraday rotation will change, thereby decreasing the isolation. To regain maximum isolation, the output polarizer can be rotated to "re-center" the isolation curve. This rotation causes transmission losses in the forward direction that increase as the difference between the usage wavelength and the design wavelength grows.
Adjustable Broadband Isolator
The isolator is set for 45° of rotation at the design wavelength. There is a tuning ring on the isolator that adjusts the amount of Faraday rotator material that is inserted into the internal magnet. As your usage wavelength changes, the Faraday rotation will change, thereby decreasing the isolation. To regain maximum isolation, the tuning ring is adjusted to produce the 45° of rotation necessary for maximum isolation.
Fixed Broadband Isolator
A 45° Faraday rotator is coupled with a 45° crystal quartz rotator to produce a combined 90° rotation on the output. The wavelength dependences of the two rotator materials work together to produce a flat-top isolation profile. The isolator does not require any tuning or adjustment for operation within the designated design bandwidth.
Tandem isolators consist of two Faraday rotators in series, which share one central polarizer. Since the two rotators cancel each other, the net rotation at the output is 0°. Our tandem designs yield narrowband isolators that may be fixed or adjustable.
Polarizer Types, Sizes, and Power Limits
Thorlabs designs and manufactures several types of polarizers that are used across our family of optical isolators. Their design characteristics are detailed below. The part number of given isolator has an identifier for the type of polarizer that isolator contains. For more details on how the part number describes each isolator, see the given isolator's manual.
Optical Isolator Tutorial
An isolator's function is based on the Faraday Effect. In 1842, Michael Faraday discovered that the plane of polarized light rotates while transmitting through glass (or other materials) that is exposed to a magnetic field. The direction of rotation is dependent on the direction of the magnetic field and not on the direction of light propagation; thus, the rotation is non-reciprocal. The amount of rotation β equals V x B x d, where V, B, and d are as defined below.
Figure 1. Faraday Rotator's Effect on Linearly Polarized Light
β = V x B x d
V: the Verdet Constant, a property of the optical material, in radians/T • m.
B: the magnetic flux density in teslas.
d: the path length through the optical material in meters.
An optical isolator consists of an input polarizer, a Faraday rotator with magnet, and an output polarizer. The input polarizer works as a filter to allow only linearly polarized light into the Faraday rotator. The Faraday element rotates the input light's polarization by 45°, after which it exits through another linear polarizer. The output light is now rotated by 45° with respect to the input signal. In the reverse direction, the Faraday rotator continues to rotate the light's polarization in the same direction that it did in the forward direction so that the polarization of the light is now rotated 90° with respect to the input signal. This light's polarization is now perpendicular to the transmission axis of the input polarizer, and as a result, the energy is either reflected or absorbed depending on the type of polarizer.
Figure 2. A single-stage, polarization-dependent isolator. Light propagating in the reverse direction is rejected by the input polarizer.
The Forward Mode
In a dual-stage isolator, the light exiting the output polarizer is sent through a second Faraday rotator followed by an additional polarizer in order to achieve greater isolation than a single-stage isolator.
The Reverse Mode
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Figure 3. A single-stage, polarization-independent isolator. Light is deflected away from the input path and stopped by the housing.
Polarization-Independent Fiber Isolators
The Forward Mode
In a dual-stage isolator, the light then travels through an additional Faraday rotator, half-wave plate, and birefringent beam displacer before reaching the output collimating lens. This achieves greater isolation than the single-stage design.
The Reverse Mode
Figure 4. Pulse Dispersion Measurements Before and After an IO-5-780-HP Isolator
τ: Pulse Width Before Isolator
τ(z): Pulse Width After Isolator
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Custom Isolator Example
Custom Adjustable Narrowband Isolator with Different Input and Output Polarizers Optimized for 650 nm Wavelength and 40 °C Temperature.
OEM Application Services
OEM and Non-Standard Isolators
In an effort to provide the best possible service to our customers, Thorlabs has made a commitment to ship our most popular free-space and fiber isolator models from stock. We currently offer same-day shipping on more than 90 isolator models. In addition to these stock models, non-stock isolators with differing aperture sizes, wavelength ranges, package sizes, and polarizers are available. In addition, we can create isolators tuned for specific operating temperatures and isolators that incorporate thermistors with heating or cooling elements for active temperature control and monitoring. These generally have the same price as a similar stock unit. If you would like a quote on a non-stock isolator, please fill out the form below and a member of our staff will be in contact with you.
Thorlabs has many years of experience working with OEM, government, and research customers, allowing us to tailor your isolator to specific design requirements. In addition to customizing our isolators (see the OEM Application Services list to the right), we also offer various application services.
We are able to provide a wide range of flexibility in manufacturing non-stock, free-space isolators. Almost any selection of specifications from our standard product line can be combined to suit a particular need. The table to the right shows the range of specifications that we can meet.
We offer isolators suitable for both narrowband and broadband applications. The size of the housing is very dependent on the desired maximum power and aperture size, so please include a note in the quote form below if you have special requirements.
We offer Faraday rotators center wavelengths from 532 nm to 1550 nm. These are the same components used to make our isolators and rotate the polarization of incoming light by 45°. Please contact Tech Support if you require a faraday rotator with a rotation angle or center wavelength outside of the aforementioned specifications.
Thorlabs is uniquely positioned to draw on experience in classical optics, fiber coupling, and isolators to provide flexible designs for a wide range of fiber optic specifications. Current design efforts are focused on increasing the Maximum power of our fiber isolators at and near the 1064 nm wavelength. We offer models with integrated ASE filters and taps. The table to the right highlights the range of specifications that we can meet.
The fiber used is often the limiting factor in determining the Maximum power the isolator can handle. We have experience working with single mode (SM) and polarization-maintaining fibers (PM); single-, double- and triple-clad fibers; and specialty fibers like 10-to-30 µm LMA fibers and PM LMA fibers. For more information about the fiber options available with our custom isolators, please see the expandable tables below.
In the spectral region below 633 nm, we recommend mounting one of our free-space isolators in a FiberBench system. A FiberBench system consists of pre-designed modules that make it easy to use free-space optical elements with a fiber optic system while maintaining excellent coupling efficiency. Upon request, we can provide select stock isolators in an optic mount with twin steel dowel pins for our FiberBench systems, as shown to the left.
We are also in the process of extending our fiber isolator capabilities down into the visible region. For more information, please contact Technical Support.
Custom Fiber Isolator
Custom Free-Space Isolator for Wavelengths Below 633 nm
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Twin Steel Pins Insert into FiberBench
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Make to Order Options
The expandable tables below provide information on some common isolator and rotator specials we have manufactured in the past. We keep the majority of the components for these custom isolators in stock to ensure quick builds, so these specials are available with an average lead time of only 2-4 weeks. Please use the Non-Stock Isolator Worksheet below for a quote.
Request a custom isolator quote using the form below or by contacting us for more information at (973) 300-3000.
I2300C4 Mechanical Drawing
I2500C4 Mechanical Drawing
I3400W4 Mechanical Drawing
I4500W4 Mechanical Drawing
I9400W4 Mechanical Drawing